There is a need for miniaturized plasma sources that can be integrated in portable or other devices for many applications such as bio-sterilization, small scale materials processing and microchemical analysis systems. Portable operation of microplasma sources places a limit on the amount of power and the vacuum levels that can be employed as well as on the maximum temperature the discharge can reach. For portable applications it is desirable to operate the discharge source at atmospheric pressure in order to eliminate the need for vacuum pumps. The temperature of the atmospheric discharge should remain low to prevent erosion and/or melting of the source. In view of the small dimensions of a miniaturized plasma source, even damage on the order of microns can become catastrophic and render the source inoperable in a short period of time.
A miniaturized inductively coupled plasma source is described in U.S. Pat. No. 5,942,855, assigned to the same Assignee as the present invention. This plasma source includes a substrate having an electrical circuit disposed thereon which includes a planar inductive coil and a capacitor coupled in series with the coil and a drive circuit coupled to the coil for driving the circuit at resonance. A plasma chamber is provided in proximity to the coil and containing a gas which is excited by energy from the coil. This source operates well but has a relatively low Q of the order of about 40, which results in lower power efficiency.
A microwave plasma source is the subject of an article entitled “A New Low-Power Microwave Plasma Source Using Microstrip Technology For Atomic Emission Spectrometry” A. M. Bilgic et al., Plasma Sources Sci. Technol 9 (2000)1-4, and an article entitled “A Low-Power 2.45 GHz Microwave Induced Helium Plasma Source At Atmospheric Pressure Based On Microstrip Technology” A. M. Bilgic et al. J. Anal. At. Spectrom. 2000, 15, 579-580. The plasma sources described in these articles create an electric field across a gap between a microstrip line on one side of a dielectric and a ground plane on the opposite side of the dielectric and wherein the gap is defined by the dielectric thickness of the device, which typically is in the range of 0.5-1 mm. The structure is not resonant and a relatively larger power input is required to initiate a plasma. In addition, the structure is susceptible to failure as ions are accelerated by a plasma sheath voltage that forms between the plasma and the microstrip line. As a result the microstrip electrode must be protected with a dielectric such as sapphire or glass. Ion erosion inherent in the design limits the usable lifetime of the device and wastes power, as power is expended in the ion erosion process rather than in the intended plasma generation.
A microwave plasma generator for a high pressure high intensity discharge lamp is disclosed in U.S. Pat. No. 5,070,277 which employ a microstrip transmission line on a low K dielectric material to drive helical coils on respective ends of a large capsule or lamp tube in which a hot plasma is formed. The device is relatively large and has a relatively large (several cm) discharge gap in which a large area hot discharge is formed. A gas mixture is sealed within the lamp tube and once heated reaches 1-10 atmospheres.